Superheterodyne receiver



May 30, 1933- w. s. SARDI-:N

SUPERHETERODYNE RECEIVER Filed June 18. 1931 G imm.

INVENTOR VLLUM S BARDEN hmm ATTORNEY lllll mum Patented May 30, 1933UNITED STATES PATENT OFFICE WILLIAM S. BARDEN, OF STAPLETON, STATENISLAND, NEW YORK, ASSIGNOR TO RADIO CORPORATION OF AMERICA, ACORPORATION OF DELAWARE SUPERHETERODYNE RECEIVER Application filed June18,

My present invention relates to radio receivers, and more particularlyto a method of, and means for, reducing interference in the operation ofa superheterodyne receiver.

A type of interference that is difficult to eliminate when using a radioreceiver comprises an interfering component, or components, falling onone side of a desired carrier, either above or below. For example,assuming that the desired signal carrier is 900 kilocycles, and themodulation com ponents carried in the lower and upper side ands (895 to905 kilocycles inclusive), then interference components disposed ineither the upper or lower side bands are difficult to eliminate.Usually, however, there will exist lesser interference in one of theside bands.

Now, I have discovered a method of, and devised means for, eliminatingside band interference by excluding at the radio receiver that side bandwith the greater interference, and conveniently retaining the side bandincluding the lesser interference.

Accordingly, it is one of the main objects of my present invention toprovide a receiving arrangement wherein, at the receiver, the side bandincluding the lesser interference may be conveniently retained, andwherein the remaining side band is excluded, means being providedwhereby the interfering component, or components, may be a parentlyshifted from one side band to t e other side band.

Another important object of the present invention is to provide in asuperheterodyne receiver an arrangement for suppressing one of the sidebands of the intermediate band of frequencies, and a means forselectively positioning interference components to be eliminated in thesuppressed band.

Another object of the present invention is to provide in asuperheterodyne receiver including a frequency changing tube, anintermediate frequency amplifier and a second detector, means positionedbetween the frequency changing tube and the second detector forsuppressing one of the side bands of the intermediate frequency band,and local oscillation means adjustable to dispose unde- 1931. Serial No.545,211.

sired interference frequencies in the side band to be su pressed.

Still other o Jects of the present invention are to improve generallythe efficiency of superheterodyne receivers, and to particularly providea receiver of this type which is free from interference residing in adesired band of signal frequencies.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically one circuit organization whereby my invention may becarried into effect.

In the drawing,

Fig. 1 diagrammatically represents a superheterodyne receiver embodyingthe present invention,

Fig. 2 is a graphical representation of the problem sought to be solved,

Figs. 3 and 4 graphically represent the shifting of the side bandinterference,

Fig. 5 shows the characteristic of the side band suppressing meansemployed in Fig. 1.

Referring to the accompanying drawing in which like characters ofreference indicate the same parts in the dierent figures, there is shownin Fig. 2 a graphical representation of an interfering componentdisposed on one side of the desired carrier frequency. Thus, the fullline at 900 kilocycles represents the carrier, while the two remainingshorter full lines, on each side of the carrier line, represent theattendant side bands extending from 895 to 905 kilocycles inclusive, thewidth of the usual broadcasting band. The interfering component is shownin dotted line, and is situated at 901 kilocycles. Of course, theinterference may, also, be located in the lower side band, or even be inboth side bands; but, assume the case where the greater interference issituated at 901 kilocycles in the upper side band. In Figs. 3 and 4there are shown graphically the action of a usual superheterodynereceiver on a desired broadcast band. As is well known, the radiofrequency carrier and associated side bands may be lowered in frequencyby a frequency changing device, as the first detector tube.

Considering Fig. 1, the antenna circuit A, G impresses the collectedenergy upon the input of a. tunable radio frequency amplifier. Thevariable condenser representation is to be understood as the means fortuning one, or more, stages of the amplifier to the desired carrierfrequency, 900 kilocycles, in Fig. Q. The local oscillator may beadjusted by'the variable condenser 2 to produce oscillations of afrequency above, or below, the carrier. In either case the firstdetector produces in its output the difference, or intermediate,frequency to be. amplified and again detected for utilization.

Assume that the intermediate frequency to be employed is 100 kilocycles.then Fig. 3 shows the desired signal carrier positioned at 100kilocycles while its associated lower and upper side bands extend from95 to 105 kilocycles inclusive. This. is. also. true in Fig. 1. If thelocal oscillator is adjusted to produce oscillations at S00 lfilocyclesso as to secure the intermediate frequency of 100 kilocycles. it isobvious that the interference component which is located at 901kilocycles will be situated in the upper side band of the L'intermediate frequency band at 101 kilocycles.

On the other hand, if t-he local oscillator is adjusted to 1000kilocycles, then the interference component is situated in the lowerside band of the intermediate frequency band at 99 ltilocycles. In otherwords, by adjusting the local oscillator to either of its two possiblesettings. there is provided a means for shifting an interferencecomponent from one side band to another side band, the shift beingapparent only in the intermediate frequency band.

Now, by disposing a low pass filter between the first and seconddetectors. which filter has a characteristic as shown in Fig. 5, it ispossible to suppress substantially completely the upper side band of theintermediate frequency band. For example, a low pass filter. possessingthe characteristic shown in Fig. 5, may be disposed between the firstdetector and the intermediate frequency amplifier, and another suchfilter B may be located between the intermediate frequency amplifier andthe second detector. In this way, there may be secured completesuppression of the upper side band. although it is to be understood thatfor the purposes of the present invention even one of the low passfilters may be used by itself.

It will be seen from Fig. 5 that the characteristic of the low passfilter is such that there is secured a sharp cut-off at the intermediatefrequency, namely 100 kilocycles. Clearly with an interference componentin the upper side band of the radio frequency band. the local oscillatorshould be adjusted to operate at 800 kilocycles so that the interferencecomponent will remain in the upper side band, and be suppressed whentransmitted through the low pass filters A and B. This should be clearfrom Fig. 4 where there is shown a shifting of the interferencecomponent into the lower side band after local oscillations of 1000kilocycles are employed. Since the lower side band is passed by the lowpassv filter. obviously the local oscillator, for the situation shown inFig. 2 should be operated at 800 kilocycles.

On the other hand, if the greater interference existed in the lower sideband in Fig. 2, for example at 899 kilocycles, then the local oscillatorwould be operated at the upper local oscillation, namely 1000kilocycles, be-

cause in that case the interference component would be shifted into theupper side band at 101 kilocycles whereby it would be suppressed whentransmitted through the low pass filter. The same treatment can beapplied, of course, to more than one interference component situated ineither of the side bands.

Those skilled in the art clearly understand the manner in which a lowpass filter having a characteristic shown in Fig. 5 is to be designed.For example, Campbell in United States Patent 1,227,113 issued May 22,1917. and United States Patent 1,493,600 issued May 13, 19:24 disclosesmethods for designing such filters.

The sharpness of eut-off is increased by using inductances andcapacities in the low pass filter of very low resistance values, and itis again pointed out that the purpose and intent in this invention isthat filters with sharp cut-ofi' characteristics shall be employed.lllitliout going into further details as to the specific construction ofthe filters it is explained that a low pass filter comprises a series ofelements consisting of inductances in series and capacities in shuntwith each other. the design being such that all frequencies below acertain value (in this case 100 kilocycles) are transmitted, but allfrequencies above the critical frequency are suppressed.

Of course. instead of using a low pass filter in the present invention,it is entirely feasible to employ a high pass filter in which case thelocal oscillator would be so adjusted that an interference component orcomponents, would be shifted into the lower side band to be suppressed.Even a band pass filter may lie employed between the first and seconddetectors, a similar method of operation being followed in such case.

lVhile I have indicated and described one system for carrying myinvention into effect it will be apparent to one skilled in the art thatmy invention is by no means limited to the particular organization shownand described, but that many modifications in the circuit arrangements,as well as in the apparatus employed, may be made Without departing fromthe scope of my invention as setforth in t-he appended claims.

`What I claim is:

l. A method of operating a superheterodyne receiver which consists incollecting a signal carrier and associated side bands, one of the sidebands including at least onentijference component, simultaneouslyreducing the carrier andside bands to an intermediate frequency carrierand associated side bands and shifting the interference component to theintermediate frequency side band opposite to its corresponding signalcarrier side band, and suppressing the intermediate frequency side bandcontaining the shifted interference component.

2. A superheterodyne receiver, comprising means for collecting a signalcarrier and associated side bands, one of which side bands includes atleast one undesirable interference component, means for reducing thesignal carrier and its side bands to intermediate frequencies, a secondfrequency reducing means, means disposed between the two frequencyreducing means for suppressing the intermediate frequency side bandcontaining the undesirable interference component, and means forshifting the undesirable interference component from its position in thecollected band to an opposite position in the intermediate frequencyside band which is to be suppressed.

3. A superheterodyne receiver, comprising means for collecting a signalcarrier and associated side bands, one of which side bands includes atleast one undesirable interference component, means for reducing thesignal carrier and its side bands to intermediate frequencies, a secondfrequency reducing means, means disposed between the two frequencyreducing means for suppressing the intermediate frequency side bandcontaining the undesirable interference component, and means includingan adjustable local oscillator for shifting the undesirable interferencecomponent from its position in the collected band to an oppositeposition in the intermediate frequency side band which is to besuppressed.

4. A superheterodyne receiver, comprising means for collecting a signalcarrier and associated side bands, one of which side bands includes atleast one undesirable interference component, means for reducing thesignal carrier and its side bands to intermediate frequencies, a secondfrequency reducing means,means comprising at least one low pass filterdisposed between the two frequency reducing means for suppressing theintermediate frequency side band containing the undesirable interferencecomponent, and means for shifting the undesirable interference componentfrom its position in the collected band to an opposite position in theintermediate frequency side band which is to be suppressed.

5. A superheterodyne receiver, comprising means for collecting a signalcarrier and associated side bands, one of which side bands includes atleast one undesirable interference component, means for reducing thesignal carrier and its side bands to intermediate frequencies, a secondfrequency reducing means, means comprising at leastone filter having asharp cut-off characteristic at the intermediate frequency disposedbetween the two frequency reducing means for suppressin the intermediatefrequency side band containing the undesirable interference component,and means for shifting the undesirable interference component from itsposition in the collected band to an opposite position in theintermediate frequency side band which is t be suppressed.

6. A superheterodvne receiver, comprising means for collect-ing a signalcarrier and associated side bands, one of which side bands includes atleast one undesirable interference component, means for reducing thesignal carrier and its side bands to intermediate frequencies, a secondfrequency reducing means, means comprising at least one low pass lterhaving a sharp cut-off characteristic at the intermediate frequencydisposed between the two frequency reducing means for suppressing theintermediate frequency side band containing the undesirableint-erference component, and means for shifting the undesirableinterference component from its position in the collected band to anopposite position in the intermediate frequency side band which is to besuppressed.

7. A superheterodyne receiver, comprising means for collecting a signalcarrier and associated side bands, one of which side bands includes atleast one undesirable interference component, means for reducing thesignal carrier and its side bands to intermediate frequencies, a secondfrequency reducing means, means comprising at least one filter having asharp cut-off characteristic at the intermediate frequency disposedbetween the two frequency reducing means for suppressing theintermediate frequency side band containing the undesirable interferencecomponent, and means including an adjustable local oscillator forshifting the undesirable interference componentfrom its position in thecollected band to an opposite position in the intermediate frequencyside band which is to be suppressed.

WILLIAM S. BAR-DEN.

